The crystal structure of cronstedtite-1T was refined in space group P31m, using two crystals: a triangular tabular crystal from Herja, Romania and a conical crystal from Lostwithiel, Cornwall, England. The Herja sample has the composition of (Fe2+2.20Fe3+0.80) (Si1.20Fe3+0.78Al0.02)O5(OH)4 and the Lostwithiel sample has the composition of (Fe2+2.32Fe3+0.68) (Si1.32Fe3+0.66Al0.02)O5(OH)4. The results of refinements are as follows: a = 5.512(1) Å, c = 7.106(1) Å, R = 3.07%, and 342 independent reflections; and a = 5.503(1), c = 7.104(1) Å, R = 2.24%, and 335 independent reflections for the Herja and Lostwithiel samples, respectively. The structure consists of one tetrahedral and one octahedral sheet. There is one octahedral site, M1, occupied by Fe only, and one tetrahedral site, T1, occupied by Si and Fe in the ratio of 0.617(8):0.383 (Herja) and 0.699(6):0.301 (Lostwithiel). Positions of two hydrogen atoms were determined from a difference map for the Lostwithiel data. The ditrigonalization angle of the tetrahedral sheet is α = −11.5° (Herja) and α = −11.1° (Lostwithiel), and the structures have a Franzini-layer type of B. The crystals studied are affected by ±b/3 stacking faults which produced slight streaking of hk ≠ 3n reflections.

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